Unpredictable rainfall, increases in soil salinity, and low temperature at the beginning or end of the growing season often result in decreased plant growth and crop productivity. These three environmental factors share at least one element of stress, and that is water deficit or dehydration.
Drought is a significant problem in agriculture today. Over the last 40 years, for example, drought accounted for 74% of the total U.S. crop losses of corn (U.S. Department of Agriculture, 1990. Agricultural Statistics. U.S. Government Printing Office, Washington, D.C.). To sustain productivity under adverse environmental conditions, it is important to provide crops with a genetic basis for coping with water deficit, for example, by breeding water retention and/or drought tolerance mechanisms into crops so that they can grow and yield under these adverse conditions.
When the rate of transpiration exceeds that of water uptake or supply, water deficit occurs and wilting symptoms appear. The responses of plants to water deficits include leaf rolling and shedding, stomata closure, leaf temperature increases, and wilting. Metabolism is also profoundly affected. General protein synthesis is inhibited and significant increases in certain amino acid pools, such as proline, become apparent (Barnett et al., 1966). During these water deficit periods, the photosynthetic rate decreases with the ultimate result of loss in yield (Boyer, 1976). If carried to an extreme, severe water deficits result in death of the plant.
Moreover, fresh water is increasingly becoming a scarce and threatened resource in large part due to agricultural production (Serageldin, 1995). Some studies have suggested that partial reduction in stomatal apertures could optimize CO2 and H2O exchange, particularly in light of rising atmospheric CO2 levels (Morison et al., 1987) and thus optimize CO2 flow into leaves for photosynthesis and water loss through transpiration. Classical studies showed that light-induced stomatal opening is mediated by K+ and anion accumulation in guard cells
A network of ion channels in the plasma membrane and the vacuolar membrane of guard cells, which controls stomatal movements, has been characterized. These ion channels are targets of early signaling branches and provide probes to identify and characterize upstream transducers. Guard cell signaling integrates water status, hormonal stimuli, light, CO2 levels and other environmental conditions to regulate stomatal apertures. Thus, guard cells are a well-developed model system for understanding how components interact within a signaling network in a single cell as they respond at the single cell level to physiological stimulation, allowing cell biological analyses in response to diverse stimuli. Guard cells respond to most of the classical plant hormones, which illustrates that unidentified receptors and early signaling mechanisms function in these cells.
Abscisic acid (ABA) mediates stress tolerance responses in higher plants, and is a key signal compound that regulates stomatal pore closure in leaves, a process requiring ion channel modulation by cytoplasmic proteins (Schroeder et al., 2001). Several soluble signaling proteins have been proposed to regulate guard cell ABA signal transduction (Schroeder et al., 2001). For example, the Arabidopsis farnesyltransferase (FTase) β subunit, encoded by ERA1, has been shown to be a negative regulator of ABA signal transduction in seeds and guard cells (Pei et al., 1998). Deletion of the ERA1 gene in Arabidopsis causes ABA hypersensitivity of both anion channel activation and stomatal closing, and during drought treatment era1 plants show reduced wilting and enhanced stomatal closure. Notably, when era1 plants are watered, the stomatal pores open, although to a lesser extent than in wild type, allowing CO2 influx and growth. Intragenic suppressor mutants of abil-1 (abil-1R) also show reduced water loss during drought; however, era1 and abil-1R alleles have additional growth and developmental phenotypes, e.g., ERA1 null mutant plants also demonstrated slowed growth (Pei et al. 1998).
Thus, downregulation of negative regulators or upregulation of positive regulators in guard cells using guard cell-specific and stress-responsive promoters, or isolation of guard cell-specific mutants, may allow engineering of plants that lose less water during drought. Some mutations may constitutively reduce stomatal opening, which could reduce the water requirements of horticultural plants and turf grass, or in agricultural regions of marginal fresh water availability, or during drought so as to slow desiccation and damage. For plants growing in humid regions, weakening of ABA signal transduction components may enhance stomatal opening and CO2 intake for carbon fixation and growth (Schroeder et al., 2001). However, conventional high-yield breeding approaches may have contributed to selection of crop plants with reduced stomatal ABA responsiveness, because genes controlling guard cell signaling are also expressed in other tissues and control other yield parameters.
Thus, what is needed is the further identification of plant genes, e.g., monocot genes, that are negative regulators of ABA signal transduction, which are useful to engineer drought hardiness into plants.